Computer Programming Concepts and Visual Basic David I. Schneider

64 Computer Programming Concepts and Visual Basic
■ GENERATING RANDOM NUMBERS: RND
Consider a specific collection of numbers. We say that a process selects a number at random
from this collection if any number in the collection is just as likely to be selected as any other
and the number cannot be predicted in advance. Some examples follow:
Collection Process
1, 2, 3, 4, 5, 6 toss a die
0 or 1, toss a coin: 0 = tails, 1 = heads
–1, 0, 1, . . . , 36 spin a roulette wheel (interpret –1 as 00)
1, 2, . . . , n write numbers on slips of paper, pull one from hat
numbers from 0 to 1 flip the spinner in Figure 2-30
FIGURE 2-30 Spinner to Randomly Select a Number Between 0 and 1
The function Rnd, which acts like the spinner in Figure 2-30, returns a random number.
The statement
picBox.Print Rnd
randomly displays a number from 0 up to (but not including) 1. The statement
numVar = Rnd
randomly assigns a number between 0 and 1 to the variable numVar. A different number will
be assigned each time Rnd is called in the program, and any number greater than or equal to
0 and less than 1 is just as likely to be generated as any other. Therefore, although Rnd looks
like a numeric variable, it does not act at all like a variable.
With appropriate scaling, the Rnd function can generate random numbers from other
collections. The statement
picBox.Print Int(6 * Rnd) + 1;
displays a number from the set 1, 2, 3, 4, 5, 6. Because Rnd always has a value from 0 to 1,
excluding 1, 6 * Rnd has a value from 0 to 6 (excluding 6), and Int(6 * Rnd) has one of the
values 0, 1, 2, 3, 4, 5. Adding 1 shifts the resulting number into the desired range.
Suppose the preceding statement is repeated many times. The integers generated should
exhibit no apparent pattern. They should look very much like a sequence of integers obtained
from successively rolling a die. For instance, each of the six integers should appear about
one-sixth of the time and be reasonably spread out in the sequence. The longer the sequence,
the more likely this is to occur.
Rnd normally generates the same sequence of numbers each time a program is run.
However, Visual Basic has another function, Randomize, that changes the sequence of numbers
generated by Rnd. This statement will be used in all programs in this text.